1. Field of the Invention
The present invention relates to the measurement of speed of a moving objective that slides on a frozen surface, such as snow, ice or similar surfaces. It is applicable to the measurement of the speed of a ski, ice skate, luge, bobsled, snowboard or similar equipment.
2. Background of the Invention
A first category of devices for measuring speed consists of devices that require external equipment previously installed. The external equipment can be, for example, a radar, a camera or an installation of photocells. The later, well know in the area of skiing, allows the determination of the average speed of the skier between two gates where photocell traps are installed.
There also exist devices for measuring speed, which use a means of positioning by triangulation, such as GPS (Global Positioning System). The position derived is accurate to several meters and the altitude derived is not very accurate. These systems are not well adapted to the determination of the speed of a skier over distances that are relatively short. In addition, the acquisition time is relatively long and obstacles can block the signal.
The devices in this first category require external equipment that is general costly and which must be installed in advance.
A second category of devices for measuring speed consists of devices that do not require an external infrastructure—they are autonomous. Included among these devices are, RPM counters that permit the determination of the speed of a bicycle or a car. For a runner there is the pedometer. The distance between the legs of a runner is proportional to the angle formed by the legs. By summing these distances one determines the speed of the runner. Contrary to the cyclist or runner, the skier doesn't have anything that makes a repetitive motion permitting measurements in such a fashion.
Other autonomous devices exist for the measurement of speed using information provided by one or more accelerometers. The detected acceleration values permit the calculation of the change in speed between two points in time. These devices require relatively complex computation. In addition, the determination of instantaneous speed requires knowledge of the history of speed changes from the start. Any error is therefore cumulative. In addition, these devices are poorly adapted to the measurement of the speed of a ski that is exposed to shock and vibration.
The German Patent DE 195 24 842 describes a device for the measurement of the speed of a ski. The measurement device contains a source of light that lights the surface of the snow passing under an opening formed in the ski. Two sensors placed in the opening receive the reflected light from two distinct zones of the lighted surface. The surface of the snow passing under the ski, having an irregular structure, causes the intensity of the light received by each sensor to fluctuate. The two sensors are placed one behind the other along the axis of the ski in such a way that the form of the signals received by each sensor is practically identical except shifted in time. The shift between the two varies as a function of the speed of the ski, the faster the ski the smaller the time difference and the slower the ski the greater the time difference. Some means of analysis, attached to the ski, determines the speed through a method of correlation using the time difference of the two signals received by each of the sensors. Knowing the space between the two sensors, the device calculates the instantaneous longitudinal speed, which is the distance between the sensors divided by time between the signals. The patent does not indicate how skidding is treated. Skidding is very common in a ski and the determination of speed is particularly important in such situations. In addition, the device in the Germany patent does not function when the ski is inclined and no longer in flat contact with the surface of the snow and when particles of snow are being projected under ski, hiding the surface from the sensors.
It is an object of the present invention is to provide a simple autonomous measurement system for longitudinal and lateral speed of a ski.
It is another object of the present invention is to provide this autonomous system capable of measuring longitudinal speed in the axis of the ski even during a skid.
It is another object of the present invention is to provide this autonomous system capable of measuring the speed of a ski no matter how it is inclined.
It is another object of the present invention is to provide this autonomous system capable of detecting the skidding of the ski.